1. Field of the Invention
This invention relates to the production of olefins from a lower aliphatic alcohol, its corresponding ether or mixtures thereof. More particularly, it relates to the catalytic conversion of a methanol feed to an olefinic product. This invention especially relates to improvements in the conversion of a methanol feed to an olefinic product in a fixed bed adiabatic reactor system.
2. Description of the Prior Art
The petrochemical industry has undergone tremendous growth in the past few decades. The production of synthetic fibers, plastics and petrochemicals by this and allied industries has steadily grown, at least in part, because of the availability of increasing supplies of inexpensive petrochemical raw materials, such as ethylene, propylene and other olefins. The principal source of ethylene at the present time is from steam cracked petroleum naphtha. The manufacture of polyethylene and styrene monomer utilizes a significant portion of this ethylene feed.
The ever-increasing demand for olefinic feedstocks has periodically caused a shortage of these basic raw materials either because of a limitation in petroleum feedstocks of suitable quality or a limitation in naphtha cracking capacity. An alternate source of ethylene from non-petroleum sources is one obvious means of keeping pace with the demand for ethylene and other olefins.
In recent years the patent art has disclosed that methanol and/or dimethyl ether, which may be obtained from coal, natural gas or biomass, can be converted to more complex hydrocarbons, such as olefins and aromatics, by utilizing a novel group of zeolites, exemplified by ZSM-5 zeolites. Ethylene is one of the olefinic hydrocarbons which may be obtained in this catalytic conversion. The reaction is highly exothermic and the olefins initially formed have a tendency to undergo further reaction to produce aromatic hydrocarbons useful in the production of motor gasoline. A large body of this patent art is concerned with various aspects of the conversion of methanol and/or dimethyl ether to light olefins, particularly ethylene.
The production of olefins from aliphatic ethers by catalytic conversion with a HZSM-5 zeolite catalyst is disclosed in U.S. Pat. No. 3,894,106 of Chang et al.
U.S. Pat. No. 3,979,472 of Butter discloses the conversion of lower alcohols and their ethers with a composite of antimony oxide and a ZSM-5 zeolite to produce a mixture of ethylene, propylene and mononuclear aromatics. U.S. Pat. No. 4,025,572 of Lago discloses that ethylene selectivity can be improved by diluting ZSM-5 with an inert diluent while a similar result is achieved, according to U.S. Pat. No. 4,025,575 of Chang et al, through use of subatmospheric partial pressure of the feed. Selectivity of ethylene is also improved by employing ZSM-5 zeolite in large crystal form of at least about 1 micron either alone (U.S. Pat. No. 4,025,571 of Lago) or in combination with added metals (U.S. Pat. No. 4,148,835 of Chen et al). Better selectivity is also obtained by interdispersing amorphous silica within the interior of the crystalline structure of the zeolite catalyst (U.S. Pat. Nos. 4,060,568 and 4,100,219 of Rodewald).
Although the above-described conversions perform exceptionally well and are unusually effective at converting lower aliphatic alcohols to olefinic hydrocarbons, it has been found that these conversions are exothermic to varying degrees depending on the particular reactant. For example, the amount of heat generated in the conversion of the lower alcohols to hydrocarbon product may be estimated to be in the ranges shown.
______________________________________ Heat Produced, BTU per lb of Alcohol Reactant Hydrocarbon Product ______________________________________ Methanol 1000-2000 Ethanol 200-620 Propanol 15-360 ______________________________________
While it is desirable that a reaction be exothermic, since this obviates the need for an external source of heat to drive the reaction, large heat generation loads can require substantial investment in complex reactors with extensive internal cooling means. It can be seen from the above table that the conversion of methanol, and to a lesser degree of ethanol, could be considered excessively exothermic in this regard. Furthermore, because of the inherent character and efficiency of the above-described crystalline aluminosilicate zeolite catalysts, the reaction of methanol, and to a lesser degree of ethanol, tend to be self-accelerating, thereby creating excessively hot local regions, where the reaction tends to go to completion, in the catalyst bed. In an adiabatic catalyst bed reactor, these highly exothermic reactions can result in high catalyst aging rates, and possibly cause thermal damage to the catalyst. Furthermore, such high temperatures could cause an undesirable product distribution to be obtained. Therefore, it is critical in the conversion of methanol to useful products to provide sufficient heat dissipating facilities so that temperatures encountered in any portion of the catalyst sequence are restricted within predetermined limits.
Additionally, it is generally good engineering practice to conduct reactant conversions at elevated pressures to more effectively utilize the reactor volume and attendant equipment. With a methanol charge, however, elevated pressures tend to produce increased quantities of 1,2,4,5-tetramethylbenzene (durene), an undesirable by-product, while lower pressures, e.g. less than 50 psig favor the production of light olefins.
Various techniques have been employed in controlling the exothermic heat released in the catalytic conversion of methanol: U.S. Pat. Nos. 3,931,349 of Kuo (use of light hydrocarbon diluents as heat sink for conversion of methanol to gasoline boiling products), 4,052,479 of Chang et al (operating conditions selected to restrict feed conversion to 5-25%) and 4,238,631 of Daviduk et al (riser reactor and dense fluid catalyst bed). U.S. Pat. No. 4,035,430 of Dwyer et al describes arranging the catalyst in a series of beds of increasing size with interstage quenching with methanol, dimethyl ether and/or light hydrocarbons for controlling exothermic heat. A tubular reactor is disclosed in U.S. Pat. No. 4,058,576 of Chang et al as a means of removing exothermic heat during the catalytic conversion of a lower alcohol to olefins. A two-stage conversion is employed with an alcohol dehydration catalyst utilized in the first stage and a ZSM-5 zeolite in the second stage which is a tubular reactor. In one embodiment, the ZSM-5 catalyst is located in the tubes of the reactor with a heat transfer medium passed through the shell side of the reactor. As the reaction mixture passes through the catalyst, the exothermic heat of reaction is released within the tubes.
U.S. Pat. No. 4,035,430 of Dwyer et al describes a means for controlling the exothermic heat release when converting methanol to gasoline boiling products. After the feed is converted to an equilibrium mixture in a bed of dehydration catalyst, it is passed through a series of zeolite catalyst beds of increasing size with interstage quenching with methanol, dimethyl ether and/or light hydrocarbons to remove the heat of reaction. The exothermic temperature rise in any one of the beds does not exceed about 50.degree. F. and the overall temperature rise does not exceed about 200.degree. F. No details are provided regarding if or how the relative size of the several catalyst beds influences the thermal stability of the desired reaction. Further, there is no suggestion that the procedures may be successfully employed where the alcohol feed is to be converted to predominantly olefinic hydrocarbons vis-a-vis aromatic compounds.
In U.S. Ser. No. 345,985 filed Feb. 5, 1982, it has been found that the exothermic heat generated in the conversion of methanol, its corresponding ether, DME, or mixtures of said alcohol and said ether in the presence of a zeolite catalyst can be effectively removed to provide a stable operation by providing the zeolite catalyst in a series of fixed beds with interstage cooling of the effluent from each bed and by limiting the temperature rise across each bed. In particular, the invention as disclosed therein relates to a process of converting methanol into olefinic hydrocarbon products which comprises:
(a) contacting a feed comprising methanol and water with a dehydration catalyst at elevated temperatures and exothermic reaction conditions effective to convert said alcohol to an ether-rich product, PA1 (b) contacting the ether-rich product of step (a) with at least a minimum number of fixed beds of crystalline aluminosilicate zeolite in a sequential manner at elevated temperatures, at substantially the same temperature increase across each of said fixed beds and under exothermic reaction conditions effective to convert said ether-rich product, at a predetermined conversion, to olefinic hydrocarbon products, said temperature increase across each of said fixed beds being no greater than the sensitivity parameter for the conversion of methanol to ethylene, said minimum number of said fixed beds being equal to the ratio of the total adiabatic temperature rise at said predetermined conversion for said methanol-water feed composition to said sensitivity parameter for the conversion of methanol to ethylene, said ratio being a whole number or the next highest whole number when said ratio is not a whole number, and said zeolite being a zeolite having a pore size greater than about 5 Angstrom units, a silica to alumina ratio of at least about 12 and a constraint index of about 1 to 12 or a zeolite characterized by pores, the major dimension of which is less than 6 Angstroms, further characterized by pore windows of about a size such as would be provided by 8-membered rings of oxygen atoms, and PA1 (c) cooling the reaction mixture effluent from each of said fixed beds, except the last, to reduce the temperature of said effluent by an amount substantially equal to the temperature increase across said fixed bed.
The entire contents of U.S. Ser. No. 345,985 are incorporated herein by reference.
The primary concern in an adiabatic multi-stage reactor design is the controllability of the operation. This type of reactor can be sensitive to even small changes in operating conditions, and if proper precautions are not taken, operation may become unstable.
It is an object of this invention to convert methanol, dimethyl ether (DME) or mixtures of methanol and dimethyl ether to an olefinic product in a thermally stable process.
It is an another object of this invention to provide a method of improving the stability of a multi-stage fixed bed adiabatic reactor system designed for the exothermic conversion of methanol and/or dimethyl ether to olefinic hydrocarbons in the presence of a zeolite catalyst.
The achievement of these and other objects will be apparent in the following description of the subject invention.